small boat opinions

Discussion in 'Boat Design' started by JLIMA, Oct 22, 2009.

  1. Oftengoofenoff
    Joined: Oct 2009
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    Location: Silverdale Wa.

    Oftengoofenoff Junior Member

    sorry about that reply my wifes damned cat ran across my keyboard. You still might want to look at the freeship file i sent you... at least i assume your using freeship.
  2. JLIMA
    Joined: Oct 2009
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    JLIMA crazed throttleman

    Well I'm off, starting the build, will post pics, and keep ya'll informed
  3. lewisboats
    Joined: Oct 2002
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    lewisboats Obsessed Member

    OK...what are your thoughts on the various thicknesses of the planking of the hull and the cabin and superstructure? What are weights and lever arms? What is the total CG and how are are you getting there? Have you placed the engine, the internals and the overall CG and is the design balanced?
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You're starting to sound like me Steve . . .

  5. robbie2161
    Joined: May 2009
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    robbie2161 Junior Member

    Sorry to just go back to the design process as I know you’ve started building. I just had a few questions with regard to the comments and the designs (which I like the look of!)

    Please correct me if I make a mistake here as I’m limited on time.

    Looking at the first design and this question is really for Tom

    The transom is much too narrow. It appears to be about 60% of BOA. 85% to 90% would be more normal. Lesser transom beam does improve down wind/wave tracking but at the expense of aft buoyancy. Don't want extreme in either direction.

    I agree this applies for the majority of planeing vessels, but in the case of a transitional vessel (or semi planeing). Wouldn’t you get away with a narrower transom in a transitional state? To me as long heavy items such as batteries and fuel tanks are placed intelligently low down and not too far back the weight acting further forward on the longitudinal centre of gravity will soften the ride and allow you to get away will a transom such as the designed one providing a more comfortable boat in small sharp chop and better wave surfing characteristics.

    Ad hoc

    Which hull design are you talking about the first or the second as I got a L/B of 3.07 Taking the overall length and beam? This is in the beamier range of a planeing boat. 2.88 would be a very wide boat and far from transitional. Is this what you meant by ‘very small’?

    Both the vessels have a Froude number of 0.72 and 0.76 calculated with at a speed of 12kts and speed/lwl of 0.64 and 0.79. with regards to Fn and the speed/LWL this is on borderline planeing.

    10 kts 0.61 and 0.64 and the speed/lwl 0.37 and 0.48
    With regards to the Fn this is transitional

    There are different opinions as to what the boundaries between the three types (transitional, planeing and Disp). The way to calculate it is Fn

    Generally speaking if the Fn is 0.4 or less, it is classed as a displacement craft.
    0.4 to 0.82 is the semi displacement category and 0.82 and above is classed as planeing. There are exceptions for example if you over power a displacement vessel it is possible to get it to go beyond the 0.4 boundary but at the expense of fuel efficiency. (Norman L Skene. ‘elements of yacht Design’ Seafarer books)

    Also another question is the powering aspect. Establishing that we are in the transitional area and taking into account 2.5 and 1.7 tonnes as a displacements which I have to agree is heavy for a boat of that size but it’s your boat fill it with what you want.

    Even with these high stated displacements this gives the first designed vessel a hp/disp of 41.39 and the second one a hp/disp of 53.53. Again I find this is well into the transitional category.

    With regards to excessive pounding it is the position of the people on board as well as the hull design. I did a bit of research into hull forms by installing accelerometers, from the research it is said that that 85% of the overall length from the bow is the position of minimum of vertical movement so quite far back.

    Designing a comfortable seakeeping hull form seems to be an art rather than a science at first but it can be done scientifically. I have a literature review I did on the subject of seakeeping if you would like me to pass it on, it only has about 10 to 15 documents as there isn’t much in the way of information in that area, hence my research.

    And finally what parameters did you decide upon for your hull? Interested to know great project, keep up the good work!

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